Use the txairbrake.version variable in setup.py.  This makes sure that version
reported to airbrake matches the version exposed to the user.
Use ElementTree instead of domish.

domish is not intended as a general XML library it is intended to generate
the specialized subset of XML used by XMPP.

This also refactors XML generation to operate on the eventDict instead of
just the failure which will later gives us the opportunity to propogate structed
data in the eventDict (such as request params and session data.)

We also add trial unit tests for the new XML generation.
Merge pull request #3 from dreid/link-versions

Use the txairbrake.version variable in setup.py.
Merge pull request #4 from dreid/etree-xml-generation

Use ElementTree instead of domish.